We propose to define the transcriptional organization of the genome of simian virus 40 (SV40) during productive infection of permissive cells and in transformed non-permissive cells using an ultraviolet light (UV) mapping technique developed in prokaryotic systems. Since the UV inactivation cross section for the expression of a particular gene is proportional to the distance between the transcriptional start point (promotor) and the promotor distal end of the gene, the locations of promotors can be determined by measuring UV inactivation kinetics for gene expression. We are measuring the UV inactivation kinetics for the synthesis of the SV40 late proteins using SDS polyacrylamide gel electrophoresis and for the late mRNAs using hybridization of RNA to restriction fragments of SV40 DNA immobilized on nitrocellulose filters. Assays for SV40 early RNA are being carried out using prior selection of the SV40 RNA on DNA-sepharose columns followed by filter hybridization. UV mapping is also being applied to the study of the expression of the Adenovirus 2 late genes.